Separate environment requests from runtime selections (#51221)

## What changed

Introduce `TurnEnvironmentRequest` for caller-supplied environment input and construct `TurnEnvironmentSelection` at session startup and app-server settings boundaries. Update thread startup, default environments, and inherited environment handoffs to use requests, with explicit conversions when reusing runtime selections.

Move the environment types into `protocol::environment` while retaining their `protocol` re-exports. Update test fixtures and existing tests to build request inputs separately from runtime selections.

GitOrigin-RevId: 12e7daaece95aae136afae8b055b4dbaebdffc75

codex-rs/core/src/session/thread_settings.rs

@@ -50,7 +50,7 @@ pub(super) fn prepare_update(
         turn_extension_init,
     } = overrides.into();
     let ThreadSettingsOverrides {
-        environments,
+        environments: environment_requests,
         runtime_workspace_roots,
         profile_workspace_roots,
         approval_policy,
@@ -79,7 +79,7 @@ pub(super) fn prepare_update(
             approval_policy,
             approvals_reviewer,
         },
-        environments,
+        environments: environment_requests,
         runtime_workspace_roots,
         profile_workspace_roots,
         sandbox_policy,